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| '''Kademlia''' is a [[distributed hash table]] for decentralized [[peer-to-peer]] [[computer network]]s designed by [[Petar Maymounkov]] and [[David Mazières]]<ref name="kademlia-paper">*[http://pdos.csail.mit.edu/~petar/papers/maymounkov-kademlia-lncs.pdf Kademlia: A Peer-to-peer information system based on the XOR Metric]</ref> in 2002.<ref>http://www.scs.stanford.edu/~dm/home/papers/</ref> It specifies the structure of the network and the exchange of information through [[node (networking)|node]] lookups. Kademlia nodes communicate among themselves using [[User Datagram Protocol|UDP]]. A virtual or [[overlay network]] is formed by the participant nodes. Each node is identified by a number or ''node ID''. The ''node ID'' serves not only as identification, but the Kademlia algorithm uses the ''node ID'' to locate values (usually file [[Hash function|hashes]] or keywords). In fact, the ''node ID'' provides a direct map to file hashes and that node stores information on where to obtain the file or resource.
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| When searching for some value, the algorithm needs to know the associated key and explores the network in several steps. Each step will find nodes that are closer to the key until the contacted node returns the value or no more closer nodes are found. This is very efficient: Like many other [[Distributed hash table|DHTs]], Kademlia contacts only [[Big O notation|<math>O(\log (n))</math>]] nodes during the search out of a total of <math>n</math> nodes in the system.
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| Further advantages are found particularly in the decentralized structure, which increases the resistance against a [[denial of service attack]]. Even if a whole set of nodes is flooded, this will have limited effect on network availability, since the network will recover itself by knitting the network around these "holes".
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| == System details == | |
| The first generation peer-to-peer file sharing networks, such as [[Napster]], relied on a central database to co-ordinate look ups on the network. Second generation peer-to-peer networks, such as [[Gnutella]], used flooding to locate files, searching every node on the network. Third generation peer-to-peer networks use [[Distributed hash table]]s to look up files in the network. ''Distributed hash tables'' store resource '''locations''' throughout the network. A major criterion for these protocols is locating the desired nodes quickly.
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| Kademlia uses a "distance" calculation between two nodes. This distance is computed as the [[exclusive or]] of the two node IDs, taking the result as an [[integer|integer number]]. Keys and Node IDs have the same format and length, so distance can be calculated among them in exactly the same way. The node ID is typically a large random number that is chosen with the goal of being unique for a particular node (see [[Universally unique identifier|UUID]]). It can and does happen that geographically widely separated nodes—from Germany and Australia, for instance—can be "neighbors" if they have chosen similar random node IDs.
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| Exclusive Or was chosen because it shares some properties with the [[Distance|geometric distance formula]]. Specifically:
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| * the distance between a node and itself is zero
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| * it is symmetric: the "distance" calculated from A to B and from B to A are the same
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| * it follows the [[triangle inequality]]: given A, B and C are [[Vertex (geometry)|vertices]](points) of a triangle, then the distance from A to B is shorter than (or equal to) the sum of the distance from A to C and the distance from C to B.
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| These properties provide most of the important behaviors of measuring a real distance with a significantly lower amount of computation to calculate it.<ref name="kademlia-paper" />
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| Each Kademlia search iteration comes one bit closer to the target. A '''basic''' Kademlia network with 2<sup>n</sup> nodes will only take ''n'' steps (in the worst case) to find that node.
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| === Routing tables === | |
| ''This section is simplified to use a single [[bit]], see the section [[Kademlia#Accelerated_lookups|accelerated lookups]] for more information on real routing tables.''
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| Kademlia routing tables consist of a ''list'' for each bit of the node ID. (e.g. if a node ID consists of 128 bits, a node will keep 128 such ''lists''.) A list has many entries. Every entry in a ''list'' holds the necessary data to locate another node. The data in each ''list'' entry is typically the ''IP address'', ''port'', and ''node ID'' of another node. Every ''list'' corresponds to a specific distance from the node. Nodes that can go in the n<sup>th</sup> ''list'' must have a differing n<sup>th</sup> bit from the node's ID; the first n-1 bits of the candidate ID must match those of the node's ID. This means that it is very easy to fill the first ''list'' as 1/2 of the nodes in the network are far away candidates. The next ''list'' can use only 1/4 of the nodes in the network (one bit closer than the first), etc.
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| With an ID of 128 bits, every node in the network will classify other nodes in one of 128 different distances, one specific distance per bit.
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| As nodes are encountered on the network, they are added to the ''lists''. This includes store and retrieval operations and even when helping other nodes to find a key. Every node encountered will be considered for inclusion in the ''lists''. Therefore the knowledge that a node has of the network is very dynamic. This keeps the network constantly updated and adds resilience to failures or attacks.
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| In the Kademlia literature, the ''lists'' are referred to as ''k-buckets''. ''k'' is a system wide number, like 20. Every k-bucket is a ''list'' having up to ''k'' entries inside; i.e. for a network with k=20, each node will have ''lists'' containing up to 20 nodes for a particular bit (a particular distance from itself).
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| Since the possible nodes for each ''k-bucket'' decreases quickly (because there will be very few nodes that are that close), the lower bit ''k-buckets'' will fully map all nodes in that section of the network. Since the quantity of possible IDs is much larger than any node population can ever be, some of the k-buckets corresponding to very short distances will remain empty.
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| [[Image:Dht example.png|thumb|420px|Network partition for node '''110''']]
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| Consider the simple network to the right. The network size is 2^3 or eight maximum keys and nodes. There are seven nodes participating; the small circles at the bottom. The node under consideration is node six (binary 110) in black. There are three ''k-buckets'' for each node in this network. Nodes zero, one and two (binary 000, 001, and 010) are candidates for the farthest ''k-bucket'' . Node three (binary 011, not shown) is not participating in the network. In the middle ''k-bucket'', nodes four and five (binary 100 and 101) are placed. Finally, the third ''k-bucket'' can only contain node seven (binary 111). Each of the three ''k-buckets'' is enclosed in a gray circle. If the size of the ''k-bucket'' was two, then the farthest ''2-bucket'' can only contain two of the three nodes. For example if node six has node one and two in the farthest 2-bucket, it would have to request a node ID lookup to these nodes to find the location (ip address) of node zero. Each node ''knows'' its neighbourhood well and has contact with a few nodes far away which can help locate other nodes far away.
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| It is known that nodes that have been connected for a long time in a network will probably remain connected for a long time in the future.<ref>Stefan Saroiu, P. Krishna Gummadi and Steven
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| D. Gribble. A Measurement Study of Peer-to-Peer File Sharing Systems. Technical Report UW-CSE-01-06-02, University of Washington, Department of Computer Science and Engineering, July 2001.</ref><ref>Daniel Stutzbach and Reza Rejaie. [http://www.barsoom.org/papers/imc-2006-churn.pdf Understanding Churn in Peer-to-Peer Networks] Section 5.5 Uptime Predictability, Internet Measurement Conference, Rio de Janeiro, October, 2006.</ref> Because of this statistical distribution, Kademlia selects long connected nodes to remain stored in the k-buckets. This increases the number of known valid nodes at some time in the future and provides for a more stable network.
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| When a ''k-bucket'' is full and a new node is discovered for that ''k-bucket'', the least recently seen node in the ''k-bucket'' is PINGed. If the node is found to be still alive, the new node is place in a secondary list, a replacement cache. The replacement cache is used only if a node in the ''k-bucket'' stops responding. In other words: new nodes are used only when older nodes disappear.
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| === Protocol messages ===
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| Kademlia has four messages.
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| * PING — used to verify that a node is still alive.
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| * STORE — Stores a (key, value) pair in one node.
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| * FIND_NODE — The recipient of the request will return the k nodes in his own buckets that are the closest ones to the requested key.
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| * FIND_VALUE — Same as FIND_NODE, but if the recipient of the request has the requested key in its store, it will return the corresponding value.
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| Each [[Remote procedure call|RPC]] message includes a random value from the initiator. This ensures that when the response is received it corresponds to the request previously sent. (see [[Magic cookie]])
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| === Locating nodes ===
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| Node lookups can proceed asynchronously. The quantity of simultaneous lookups is denoted by α and is typically three. A node initiates a FIND_NODE request by querying to the α nodes in its own ''k-buckets'' that are the closest ones to the desired key. When these recipient nodes receive the request, they will look in their ''k-buckets'' and return the ''k'' closest nodes to the desired key that they know. The requester will update a results list with the results (node ID's) he receives, keeping the ''k'' best ones (the k nodes that are closer to the searched key) that respond to queries. Then the requester will select these ''k'' best results and issue the request to them, and iterate this process again and again. Because every node has a better knowledge of his own surroundings than any other node has, the received results will be other nodes that are every time closer and closer to the searched key. The iterations continue until no nodes are returned that are closer than the best previous results. When the iterations stop, the best k nodes in the results list are the ones in the whole network that are the closest to the desired key.
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| The node information can be augmented with [[Round-trip delay time|round trip times]], or [[Round-trip delay time|RTT]]. This information will be used to choose a time-out specific for every consulted node. When a query times out, another query can be initiated, never surpassing α queries at the same time.
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| === Locating resources ===
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| Information is located by mapping it to a key. A [[Hash function|hash]] is typically used for the map. The storer nodes will have information due to a previous STORE message. Locating a value follows the same procedure as locating the closest nodes to a key, except the search terminates when a node has the requested value in his store and returns this value.
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| The values are stored at several nodes (k of them) to allow for nodes to come and go and still have the value available in some node. Periodically, a node that stores a value will explore the network to find the k nodes that are close to the key value and replicate the value onto them. This compensates for disappeared nodes.
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| Also, for popular values that might have many requests, the load in the storer nodes is diminished by having a retriever store this value in some node near, but outside of, the k closest ones. This new storing is called a cache. In this way the value is stored farther and farther away from the key, depending on the quantity of requests. This allows popular searches to find a storer more quickly. Because the value is returned from nodes farther away from the key, this alleviates possible "hot spots". Caching nodes will drop the value after a certain time depending on their distance from the key.
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| Some implementations (e.g. [[Kad network|Kad]]) do not have replication nor caching. The purpose of this is to remove old information quickly from the system. The node that is providing the file will periodically refresh the information onto the network (perform FIND_NODE and STORE messages). When all of the nodes having the file go offline, nobody will be refreshing its values (sources and keywords) and the information will eventually disappear from the network.
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| === Joining the network ===
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| A node that would like to join the net must first go through a [[Bootstrapping node|bootstrap]] process. In this phase, the joining node needs to know the [[IP address]] and port of another node—a bootstrap node (obtained from the user, or from a stored list)—that is already participating in the Kademlia network. If the joining node has not yet participated in the network, it computes a [[randomness|random]] ID number that is supposed not to be already assigned to any other node. It uses this ID until leaving the network.
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| The joining node inserts the bootstrap node into one of its ''k-buckets''. The joining node then does a FIND_NODE of its own ID against the bootstrap node (the only other node it knows). The "self-lookup" will populate other nodes' ''k-buckets'' with the new node ID, and will populate the joining node's k-buckets with the nodes in the path between it and the bootstrap node. After this, the joining node refreshes all ''k-buckets'' further away than the k-bucket the bootstrap node falls in. This refresh is just a lookup of a random key that is within that ''k-bucket'' range.
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| Initially, nodes have one ''k-bucket''. When the ''k-bucket'' becomes full, it can be split. The split occurs if the range of nodes in the ''k-bucket'' spans the node's own id (values to the left and right in a binary tree). Kademlia relaxes even this rule for the one "closest nodes" ''k-bucket'', because typically one single bucket will correspond to the distance where all the nodes that are the closest to this node are, they may be more than k, and we want it to know them all. It may turn out that a highly unbalanced binary sub-tree exists near the node. If ''k'' is 20, and there are 21+ nodes with a prefix "xxx0011....." and the new node is "xxx0000''11001''", the new node can contain multiple ''k-buckets'' for the other 21+ nodes. This is to guarantee that the network knows about all nodes in the closest region.
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| === Accelerated lookups ===
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| Kademlia uses an ''[[exclusive or|XOR]] [[metric (mathematics)|metric]]'' to define distance. Two node ID's or a node ID and a key are XORed and the result is the distance between them. For each bit, the XOR function returns zero if the two bits are equal and one if the two bits are different. XOR metric distances hold the [[triangle inequality]]: given A, B and C are [[Vertex (geometry)|vertices]](points) of a triangle, then the distance from A to B is shorter than (or equal to) the sum of the distance from A to C to B.
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| The ''XOR metric'' allows Kademlia to extend routing tables beyond single bits. Groups of bits can be placed in ''k-buckets''. The group of bits are termed a prefix. For an ''m-bit'' prefix, there will be 2<sup>m</sup>-1 ''k-buckets''. The missing ''k-bucket'' is a further extension of the routing tree that contains the node ID. An ''m-bit'' prefix reduces the maximum number of lookups from ''log<sub>2</sub> n'' to ''log<sub>2<sup>b</sup></sub> n''. These are '''maximum''' values and the average value will be far less, increasing the chance of finding a node in a ''k-bucket'' that shares more bits than just the prefix with the target key.
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| Nodes can use mixtures of prefixes in their routing table, such as the [[Kad Network]] used by [[eMule]]. {{Citation needed|date=January 2010}} <!-- this doesn't make sense, a prefix is a length of bits used in the 'xor metric'. The Store values can be a mixture of types. Many networks do this. Perhaps statement confusing would be a better template? --> The Kademlia network could even be heterogeneous in routing table implementations, at the expense of complicating the analysis of lookups.
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| == Academic significance ==
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| While the XOR metric is not needed to understand Kademlia, it is critical in the analysis of the protocol. The XOR arithmetic forms an [[abelian group]] allowing closed analysis. Other DHT protocols and algorithms required [[Computer simulation|simulation]] or complicated formal analysis in order to predict network behavior and correctness. Using groups of bits as routing information also simplifies the algorithms.
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| == Use in file sharing networks ==
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| Kademlia is used in [[file sharing]] networks. By making Kademlia keyword searches, one can find information in the file-sharing network so it can be downloaded.
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| Since there is no central instance to store an index of existing files, this task is divided evenly among all clients: If a node wants to share a file, it processes the contents of the file, calculating from it a number ([[hash function|hash]]) that will identify this file within the file-sharing network. The hashes and the node IDs must be of the same length. It then searches for several nodes whose ID is close to the hash, and has its own IP address stored at those nodes. i.e. it publishes itself as a source for this file. A searching client will use Kademlia to search the network for the node whose ID has the smallest distance to the file hash, then will retrieve the sources list that is stored in that node.
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| Since a key can correspond to many values, e.g. many sources of the same file, every storing node may have different information. Then, the sources are requested from all k nodes close to the key.
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| The file hash is usually obtained from a specially formed Internet [[Magnet URI scheme|link]] found elsewhere, or included within an indexing file obtained from other sources.
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| Filename searches are implemented using [[Index term|keywords]]. The filename is divided into its constituent words. Each of these keywords is hashed and stored in the network, together with the corresponding filename and file hash. A search involves choosing one of the keywords, contacting the node with an ID closest to that keyword hash, and retrieving the list of filenames that contain the keyword. Since every filename in the list has its hash attached, the chosen file can then be obtained in the normal way.
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| == Implementations ==
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| === Networks ===
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| <!-- Dont put clients here. The protocol page should list clients which presumably also have DHT capabilities. Please provide references. --> | |
| Public networks using the Kademlia algorithm (these networks are incompatible with one another):
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| * [[Kad Network]] — developed originally by the [[eMule]] community to replace the server-based architecture of the [[eDonkey2000 network]].
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| * [[Overnet|Overnet network]]: With [http://kadc.sourceforge.net/ KadC] a C library for handling its Kademlia is available. (development of Overnet is discontinued)
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| <!-- *[http://www.revconnect.com/ RevConnect] - v0.403+. Please provide proof. -->
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| * [[BitTorrent (protocol)|BitTorrent]] Uses a DHT based on an implementation of the Kademlia algorithm, for trackerless torrents.
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| * [[Osiris (Serverless Portal System)|Osiris sps]] (all version): used to manage distributed and anonymous web portal.
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| * [[Retroshare]] — F2F decentralised communication platform with secure VOIP, instant messaging, file transfer etc.
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| * [[Gnutella]] DHT — Originally by [[LimeWire]]<ref name="mojito slyck">[http://www.slyck.com/story1235.html Mojito and LimeWire]</ref><ref name="mojito wiki">[http://web.archive.org/web/20090217070609/http://wiki.limewire.org/index.php?title=Mojito Mojito Wiki (archive.org)]</ref> to augment the [[Gnutella]] protocol for finding alternate file locations, now in use by other gnutella clients.<ref>[https://gtk-gnutella.svn.sourceforge.net/svnroot/gtk-gnutella/trunk/gtk-gnutella/ChangeLog Gtk-gnutella changelog] Retrieved January, 2010.</ref>
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| <!-- *[http://retroshare.sf.net/ RetroShare] - Kademlia implementation for secure Peer-to-Peer messaging and File Sharing No information in regards to Kadmelia or what network protocol. -->
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| === Libraries ===
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| * [http://khashmir.sourceforge.net/ Khashmir] — Python implementation of Kademlia. Used in the mainline BitTorrent, with some modifications.
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| * [http://entangled.sourceforge.net/ Entangled] — Python implementation of Kademlia, also providing a distributed [[tuple space]]. LGPL licensed
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| * Mojito — a Java Kademlia library written for the [[LimeWire]] project. See the [http://www.limewire.org/nightly/modules/mojito/api/org/limewire/mojito/class-use/MojitoDHT.html Class interface documentation]{{dead link|date=October 2010}} for more information.<ref name="mojito slyck"/><ref name="mojito wiki"/>
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| * [http://code.google.com/p/maidsafe-dht/ maidsafe-dht] — c++ implementation of Kademlia (Commercial or GPL license), with NAT traversal and crypto libraries.
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| * [http://www.thomas.ambus.dk/plan-x/routing/ Plan-x] — Java implementation.
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| * [http://daylight.sourceforge.net/ Daylight] — C#/.NET implementation.
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| * [http://bitdht.sourceforge.net/ BitDHT] — C++ implementation of Kademlia. Uses BitTorrent DHT network (LGPL license)
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| * [https://github.com/rkapsi/ardverk-dht Ardverk-DHT] — Java implementation (ASL 2.0 license)
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| * [http://tomp2p.net/ TomP2P] - Java implementation with UPNP support for NAT traversal (ASL 2.0 license)
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| * [http://code.google.com/p/openkad/ openkad] - Java implementation with distributed caching (GNU GPL v3 license)
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| ==Next generation==
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| Over the years, the academic and practitioner communities have realized that all current DHT designs suffer from a security weakness, known as the [[Sybil attack]]. [http://pdos.csail.mit.edu/~petar/ Petar Maymounkov], one of the original authors of Kademlia, has proposed a way to circumvent this weakness by incorporating social trust relationships into the system design. See [http://pdos.csail.mit.edu/papers/sybil-dht-socialnets08.pdf], page 20 for social-network based approaches.
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| The new system, codenamed [http://pdos.csail.mit.edu/~petar/5ttt.org/ Tonika] or also known by its domain name as 5ttt, is based on an algorithm design known as [http://arxiv.org/abs/0909.2859 Electric routing] and co-authored with the mathematician [http://math.mit.edu/~kelner/ Jonathan Kelner]. Maymounkov has now undertaken a comprehensive implementation effort of this new system, which is entirely based on the [[Go (programming language)|Go programming language]]. However, research into effective defences against Sybil attacks is generally considered an open question, and wide variety of potential defences are proposed every year in top security research conferences.
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| == See also ==
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| * [[Content addressable network]]
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| * [[Chord (DHT)]]
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| * [[Tapestry (DHT)]]
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| * [[Pastry (DHT)]]
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| * [[Koorde]]
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| == References ==
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| {{reflist}}
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| == External links ==
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| * [http://pdos.csail.mit.edu/~petar/ Academic Home Page] of Petar Maymounkov, co-designer of Kademlia.
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| * [http://xlattice.sourceforge.net/components/protocol/kademlia/specs.html Xlattice projects] Kademlia Specification and definitions.
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| * [http://www.cs.northwestern.edu/~yqiao Yi Qiao] and [http://www.cs.northwestern.edu/~fabianb Fabian E. Bustamante] [http://www.aqualab.cs.northwestern.edu/publications/YQiao06SUO.pdf USENIX 2006 paper that characterizes Overnet and Gnutella]
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| * {{Cite web | url=http://www.barsoom.org/~agthorr/papers/infocom-2006-kad.pdf | last = Stutzbach| first = Daniel| title = Improving Lookup Performance over a Widely-Deployed DHT| publisher = University of Oregon | year = 2006}}
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| *[http://www.cs.technion.ac.il/~gilga/P2P2013_49.pdf Kaleidoscope: Adding Colors to Kademlia] IEEE P2P 2013 (cache friendly lookup for kademlia)
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| * [http://www.cs.helsinki.fi/u/jakangas/MLDHT/ Mainline DHT Measurement] at Department of Computer Science, University of Helsinki, Finland.
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| [[Category:2002 introductions]]
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| [[Category:Distributed data storage]]
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